The present invention relates to semiconductor devices using silicon oxide films and methods for forming silicon oxide films, particularly to semiconductor devices using very thin silicon oxide films, semiconductor devices including element isolation structures in which a dielectric substance is buried in silicon, semiconductor devices including element isolation structures formed on an insulating film, and methods and apparatus for forming silicon oxide films.
The gate insulating films of transistors formed on silicon substrates require high performance characteristics such as low interface level density, and high reliability such as high withstand voltage and high hot carrier tolerance. As a conventional oxide film formation technique to meet those requirements, thermal oxidation at 800xc2x0 C. or more has been used.
Besides, from a demand for forming transistors on a silicon semiconductor at higher density, for transistor integrated elements formed on a silicon substrate, in concert with the progress of scale-down technique, in place of selective oxidation film (LOCOS) element isolation structures in which bird""s beaks extend, element isolation structures such as shallow trench isolation that enables narrow dielectric isolation came to be used.
Besides, for integrated elements such as SOI (Silicon On Insulator) transistors and polysilicon transistors formed on an insulating film, used were element isolation structures in which silicon films are formed into islands by LOCOS isolation with silicon oxide films and mesa isolation by etching silicon off.
For formation of semiconductor elements with ultrahigh integration/ultrahigh speed drive, however, conventional thermal oxidation can not be used. To realize ultrahigh speed elements, a metallic material must be introduced in the semiconductor device. But, if a high temperature process at 550xc2x0 C. or more is used, the metal can react with the semiconductor to deteriorate the operation performance of the elements. Besides, if such a high temperature process is used, it becomes difficult to form an accurate impurity distribution because of rediffusion of impurities. This makes it hard to form ultrahigh integrated elements. Therefore, oxide film formation at a low temperature of 550xc2x0 C. or less is indispensable.
So, in recent years, techniques for forming silicon oxide films at low temperatures have been studied. But, characteristics of a silicon oxide film formed at 550xc2x0 C. or less were never equal to those of a thermal oxide film. The oxidation speed of such conventional low-temperature oxidation is lower than that of thermal oxidation, so electrical characteristics, such as interface level density and current-voltage characteristic, of a silicon oxide film formed were greatly inferior to those of a thermal oxide film.
Besides, in a conventional element isolation structure for transistor integrated elements formed on a silicon substrate, the thickness of a silicon oxide film at a portion near a corner of an element isolation side wall portion is smaller than that on a flat silicon surface portion. Therefore, a problem arose that characteristics such as leakage current and withstand voltage of the oxide film are inferior at the thin portion and the reliability in performance of the elements are deteriorated. Further, since a parasitic transistor element having its thin gate oxide film exists in parallel with a transistor element having its gate oxide film of a normal thickness, this deteriorated the voltage-current characteristic of the transistor.
In order to solve such problems, if the thickness of the silicon oxide film is simply increased to avoid the problem that arises at the thin portion, since the silicon oxide film also serves as a gate oxide film, a problem arises that the drive performance of the MOS transistor deteriorates. So, conventionally, the angle of the side wall portion of the recessed portion in the element isolating region with the silicon surface is set at about 70 degrees or less so that thinning of the silicon oxide film at corners of the side wall portion is relieved. Even in this case, however, about 30% or more thinning occurred, and occurrence of characteristic deterioration such as leakage current and withstand voltage of the oxide film at the thin portion could not completely be prevented. Furthermore, formation of the recessed element isolating region with an obtuse angle brought about problems that the element isolation width increased, the ratio in area of the effective region where elements such as transistors are to be formed decreased, and high density integration could not be intended.
Furthermore, in a conventional element isolation structure for integrated elements such as SOI (Silicon On Insulator) transistors and polysilicon transistors formed on an insulating film, in case of LOCOS element isolation, a parasitic transistor element existed near the interface between the element isolation oxide film below a gate electrode and silicon. This deteriorated electrical characteristics of the transistor, in particular, subthreshold current characteristic and off-leak characteristic. On the other hand, in case of mesa element isolation, a high quality oxide film could not be formed on the element isolation side wall portion where silicon has been etched off. This had a bad influence on characteristics, in particular, off characteristics, of the transistor.
Accordingly, it is an object of the present invention to provide semiconductor devices, and methods and apparatus for forming silicon oxide films, which make it possible to realize silicon oxide films having, even though they were formed by low-temperature plasma oxidation, characteristics and reliability superior to those of silicon thermal oxide films formed at a high temperature of about 1000xc2x0 C., and to realize high-performance transistor integrated circuits that can reduce the area of their element isolating regions and be suitably applied to SOI transistors and TFTs.
The present invention was made for solving those conventional problems, and a semiconductor device of the present invention includes a plurality of transistors with their substrate of silicon, and is characterized in that at least part of a silicon oxide film formed on a surface of said silicon contains Kr (krypton).
According to an aspect of the semiconductor device of the present invention, a recessed groove is formed at part of said substrate surface between said plurality of transistors, and a dielectric substance is formed in part of said groove, said silicon oxide film is formed on a corner of said substrate surface in said groove, and at least part of said silicon oxide film contains Kr.
According to an aspect of the semiconductor device of the present invention, a side wall portion in said groove is formed such that the angle of part of said side wall portion with said substrate surface exceeds at least 75 degrees.
According to an aspect of the semiconductor device of the present invention, the difference in thickness of said silicon oxide film between the portions formed on at least part of the surface other than said groove of said substrate and on at least part of the surface in said groove is within 30%.
A semiconductor device of the present invention wherein a semiconductor film at part of a surface of which a recessed groove is formed or an island-shape semiconductor film is formed on an insulating film, is characterized in that a silicon oxide film is formed on a corner of said semiconductor film of said groove or a corner of said semiconductor film, and at least part of said silicon oxide film contains Kr.
According to an aspect of the semiconductor device of the present invention, the content of Kr contained in said silicon oxide film decreases from said silicon oxide film surface toward a silicon/silicon oxide film interface.
According to an aspect of the semiconductor device of the present invention, the Kr content in said silicon oxide film is 5xc3x971011 cmxe2x88x922 or less at the surface density.
A method of the present invention for forming a silicon oxide film, is characterized by introducing a mixture gas mainly containing a gas containing oxygen and Kr gas into a process chamber, exciting plasma with a microwave, and directly oxidizing a silicon substrate surface placed in the process chamber, thereby forming a silicon oxide film on said silicon substrate surface.
According to an aspect of the method of the present invention for forming a silicon oxide film, said silicon oxide film is the gate insulating film of a transistor.
According to an aspect of the method of the present invention for forming a silicon oxide film, the oxygen partial pressure in said mixture gas is 2 to 4%, and the pressure in said process chamber is 800 mTorr (106 Pa) to 1.2 Torr (160 Pa).
According to an aspect of the method of the present invention for forming a silicon oxide film, said plasma is plasma excited with a microwave of a frequency of 900 MHz to 10 GHz.
An apparatus of the present invention for forming a silicon oxide film, is characterized by comprising a process chamber in which a silicon substrate is placed, and a waveguide tube for supplying a microwave in said process chamber, and in that a mixture gas mainly containing a gas containing oxygen and Kr gas is introduced into said process chamber, plasma is excited with the microwave, and said silicon substrate surface is directly oxidized, thereby forming a silicon oxide film on said silicon substrate surface.
A semiconductor device of the present invention includes a plurality of transistors each having its source and drain regions each comprising a high impurity concentration region, and is characterized in that at least part of the portion between said source and drain regions is a silicon oxide film containing Kr.
In the present invention, even though it was formed by low-temperature plasma oxidation, formation of a silicon oxide film having its characteristics and reliability superior to a silicon thermal oxide film formed at a high temperature of about 1000xc2x0 C. becomes possible, and a high-performance transistor integrated circuit can be realized.
In the present invention, the thickness of the silicon oxide film at a portion near a corner of an element isolation side wall portion is never thinned, and it is substantially equal to the film thickness of the flat silicon surface portion. Therefore, characteristics of the oxide film such as leakage current and withstand voltage are good, and so an improvement of reliability of the element can be realized. Besides, since this silicon oxide film is usable even in a thinned state as a gate oxide film, both of an improvement of reliability of element isolation and an improvement of drive performance of a MOS transistor can be realized at once. Besides, even if the angle of the side wall portion of the recessed portion of the element isolating region of the silicon substrate with the silicon surface is set at about 75 degrees or more or 90 degrees, thinning of the silicon oxide film at a corner of the side wall portion does not occur, and so a narrow element isolating region can be formed, the ratio in area of the effective region for forming elements such as transistors increases, and high density integration can be realized.
Further, even in an element isolation structure for SOI (Silicon On Insulator) transistors or polysilicon transistors formed on an insulating film, a high-quality oxide film can be formed on the element isolation side wall portion, and, without any parasitic transistor existing, the electrical characteristics of each transistor can be good.